In various related art image forming apparatuses, in order to fix toner onto a sheet or to dry ink, a fixing apparatus may be arranged which includes a heating unit and a pressurizing unit downstream of an image forming unit which forms an image onto the sheet. In the fixing apparatus, a nip portion which places the sheet between the heating unit and the pressurizing unit may be configured in an arc shape in order to make it possible to heat the sheet for a sufficient time and the sheet may be bent to the pressurizing member side at an outlet of the nip portion in order to make it possible for the sheet to be peeled off well from a fixing member.
Moreover, as the sheet is conveyed onto a paper-discharging tray which is arranged such that the sheet is easily taken out therefrom while keeping an installation area of an apparatus in question small, a sheet conveying path up to a point at which the sheet is discharged onto the paper-discharging tray from the fixing apparatus is configured by combining multiple bent conveying paths.
As the sheet is generally a viscoelastic body, stress is relaxed when strain is provided thereto, so that residual strain is produced. Therefore, when the sheet passes through the conveying path or the fixing apparatus including the above-described bent portion, strain remains in the bent portion, forming a curl. When a curled sheet of paper is conveyed through the conveying path and discharged, it becomes more likely for a paper jam in the conveying path after fixing and bulking, etc., in storage in the paper-discharging tray to occur. Therefore, in order to straighten such a curl which occurs in the conveying path or at the time of fixing, a curl straightening apparatus may be used on the conveying path of the sheet.
Curl straightening apparatuses are known which include a pressing roller and a roller or a belt, for example, and which straightens the curl of the sheet by causing the sheet to pass through the nip portion formed by pressing the roller or the belt by the pressing roller.
In order to properly remove the curl in such a curl straightening apparatus, it is necessary to control the amount of straightening. In this way, no technique is known which accurately projects an amount of curl produced that varies depending on operating conditions of the image forming apparatus. Therefore, a curl straightening method which makes it possible to stably reduce curl is not known since problems occur such that a curl suppressing effect is low compared to an actual curl amount or that a greater adjustment of the suppressing effect causes it to be excessive on the contrary, causing a curl on the reverse side.